1. Field of the Invention
The present invention relates to surface treatment applied to the surfaces of metal products such as tools and mechanical parts. More particularly, the present invention relates to an instantaneous heat treatment method for metal products, in which the surface of a metal product is repeatedly subjected to instantaneous rapid heating and rapid cooling by intermittently ejecting abrasive (referred to as “shot” in this description) with a mixture of three or more approximate but different particle sizes for high ejection density onto the surface of the metal product, thereby achieving both effects of forming uniform microstructures near the surface of the metal product and forming micro-diameter dimples on the metal surface.
In the description, the term “particle size” refers to a range of average particle diameter.
2. Description of the Related Art
In general, various types of surface treatment processes are employed in order to endow the surface of a metal product with intended properties.
In particular, among various metal products, in order to prevent wear on sliding portions, drive portions, and seal portions of metal products which are prone to severe wear, as well as cutting tools and molds which are especially prone to wear, the hardness of the material of such metal products has been conventionally increased by heat treatment processes, such as quenching, carburization, and nitriding. Alternatively, the surface roughness of such metal products has been conventionally improved by machining, such as cutting, grinding, and polishing with a machine tool such as a lathe or a milling machine. Moreover, an oil film has been formed on the surface at a sliding portion etc. of a metal product by an oil supply process such as an oil bath process, splash process, drop process, cycling process or spray process in order to prevent wear at the sliding portion etc., and various advances have been made including a method for effectively forming an oil film on the surface of a metal product based on these oil supply processes and a method for improving the quality of lubricating oil to prevent the oil film from running out.
Miniaturization of Metal Structures
In comparison with surface heat treatment for achieving high hardness of the surface of a metal product by the above-described known quenching method, an instantaneous heat treatment method has recently been proposed involving miniaturizing structures in the surface of a metal product by instantaneously applying rapid heating and rapid cooling to the surface of the metal product, thereby dramatically increasing the mechanical strength of the metal product. Metal deformation is caused by dislocation, and it is well-known that the smaller the metal crystal grains are, the harder the metal becomes because such dislocation is restricted due to an increased number of crystal grain boundaries as a result of the miniaturization.
Pulse quenching has been proposed as the above-described instantaneous heat treatment method applied to a metal surface. Because this pulse quenching heats a thin surface layer up to the austenitizing (γ) temperature in a very short period of time (on the order of ms) and subsequently removes the heat by means of the surrounding materials, rapid cooling can be achieved without having to use a specific coolant, thereby producing an extremely fine, hardened structure. In addition to this effect, it has also been found that microstructures can be obtained through phase transformation of locally melted portions being subjected to rapid solidification with friction heat arising from lubricated friction carried out using cast iron. Furthermore, it has been found that laser heat treatment can produce a uniform, fine martensite structure due to its heating/cooling process featuring rapid heating and rapid cooling. It has also been proposed that crystal grains be miniaturized by high-frequency quenching in which rapid heating is performed for two seconds at 950 to 1,000° C. by high-frequency induction heating, followed by rapid cooling.
In addition to instantaneous heat treatment by the above-described pulse quenching method, a method of ejecting shot onto the surface of a metal product to instantly heat the metal surface by collision of the shot and nanosizing of structures in the metal surface due to a self-quenching effect down to the room temperature is a well-known instantaneous heat treatment for miniaturizing structures in the surface layer of a metal product.
It has been found that pure iron changes to nano-crystals with a crystal grain diameter of 100 nm or smaller at depths of several μm to several tens of μm from the surface by applying shot peening treatment to pure iron under shot conditions with a particle diameter of 50 μm, an ejection speed of 190 m per second, and a treatment time of 10 seconds. It has been found that a nano-crystal structure is generated adjacent to the surface of carburized-quenched steel JIS-SCr420 by ejecting SKH59 particles with an average particle diameter of 45 μm onto the steel for 30 seconds at an air pressure of 0.5 MPa.
It has also been found that nano-crystals are formed adjacent to the surface of pure iron by ejecting shot with a particle diameter of 50 μm onto the pure iron for a treatment time of 100 seconds at an air pressure of 0.8 MPa.
Dimple Formation
Furthermore, the above-described surface treatment by ejecting shot is employed as a method of effectively forming an oil film on the surface of the metal product for the purpose of wear prevention. In more detail, a method has been proposed for preventing wear at a sliding portion of a metal product, in which substantially spherical shot having a hardness equal to or higher than the hardness of the metal product is ejected onto the surface at a sliding portion of the metal product and oil reservoirs (micropools) composed of numerous fine concavities (dimples) substantially arc-shaped in cross section are formed on the surface at the sliding portion of this metal product to prevent an oil film from easily running out (Japanese Patent No. 3212433).
It is also known that, in addition to the above-described effects of miniaturizing crystal structures and forming dimples, shot peening offers the effects of hardening the surface of a metal product, increasing the fatigue strength and so forth, because compressive residual stress is generated resulting from plastic deformation due to collision when shot is ejected onto the surface of the metal product.
Besides the above-described processes, surface treatment processes for qualitatively improving the surface of a metal product include mirror finishing that endows raw metal with abrasion resistance. Furthermore, plating and ceramic coating are performed to offer oxidation resistance, heat resistance, abrasion resistance, and corrosion resistance. Recently, ceramic coating by PVD or CVD, DLC coating and so forth are performed following surface finishing.
Unfortunately, the known ('2433) surface treatment process has the following problems associated with the above-described sliding portions, drive portions, and seal portions of metal products, which severely deteriorate due to wear, and with cutting tools and molds, which are easily worn metal products.
Sliding portions of metal products suffer from a problem in that mirror finishing for the purpose of decreasing the sliding resistance causes wringing (phenomenon in which mirror surfaces adhere to each other) due to running out of an oil film and generates high resistance, leading to a knocking sound. The term wringing indicates a phenomenon in which two end surfaces in the form of smooth flat surfaces facing each other of products such as block gages, cannot be easily separated from each other when they are pressed against each other while being slid.
What is worse, even if an attempt is made to provide a lubrication effect with oil reservoirs (micropool effect) by forming fine (micro) dimples by ejecting shot as described above, in order to effectively form an oil film on the surface at a sliding portion of a metal product, it is difficult to form such an oil film due to an increase in the sliding speed or a decrease in oil viscosity, which is intended for achieving high fuel efficiency for the purpose of saving energy.
It is becoming possible to increase the strength of drive portions of metal products, for example, through carburization/quenching, followed by shot peening. However, because of the above-described demand for high fuel efficiency, the oil viscosity is decreased, which makes it difficult to form an oil film, causing serious problems including a decrease in the service life and generation of sound and vibrations.
In the case of seal portions of metal products, especially when two faces in contact with each other perform rotational or reciprocal motion in order to form a seal face, the seal face composed of the two faces in contact with each other requires hermeticity and responsiveness in order to prevent liquid and air from leaking Crevice corrosion prevention is required for threaded portions and metal-to-metal contact portions. Conventionally, the two faces constituting the above-described seal face are subjected to mirror finishing and, in some cases, plating or coating following this mirror finishing. However, mirror finishing is problematic because it causes wringing which decreases the responsiveness between the two faces constituting the seal face thereby generating sound. There are also problems with threaded portions or metal-to-metal contact portions in that screws cannot be untightened due to crevice corrosion, valves cannot be opened due to rust or even if they are opened, they cannot be closed.
In the case of cutting tools, there are growing demands for the use of water-soluble cutting oil, decreased usage amount of such cutting oil, or even use of dry processing for environmental reasons. These demands, however, lead to a decrease in the service life of cutting tools. On the other hand, molds are required to become lighter to increase fuel efficiency. For this reason, steel plates endowed with higher strength in a cooling step by adding manganese or silicon to iron, so-called high-tension materials are being used in larger amounts, further decreasing the service life of molds. In any case, a ceramic coating is generally applied, but a satisfactory solution cannot be achieved by coating alone, due partially to the problem of coating delamination.
Miniaturization of Metal Structure
The above-described instantaneous heat treatment method for miniaturizing metal structures by heat treatment has the following problems associated with it.
First, surface heat treatment by pulse quenching is suitable for some types of steel but not for others and is thus feasible only for limited applications. For example, high-frequency surface hardening is used mainly for carbon steel but adversely affects stainless steel. Laser quenching is also limited to special types of parts.
In contrast, heat treatment by shot blasting can be applied to any types of steel. Although it succeeds in forming nano-structures, heat treatment by shot blasting damages the surface of the product to be treated depending on the treatment conditions if the product is soft, thus the product cannot be placed into practical use. Another problem with heat treatment by blasting is that the entire treated surface layer cannot be nano-structured.
Although a method of forming nano-crystals on the surface layer of carburized-quenched steel JIS-SCr420 having relatively high hardness by shot peening has also proposed, it cannot enhance the mechanical properties such as the strength (tensile strength and yield point) and the toughness (stretch, drawing, and impact value), because the crystal grains are not uniform at a depth of 0.5 μm from the surface.
On the other hand, decreased oil viscosity in order to achieve high fuel efficiency as described above, may cause an oil film to run out even though oil reservoirs in the form of concavities are formed on the surface of a metal product by the known shot blasting disclosed in '2433, thus resulting in wringing, decreased service life, sound, and vibrations. It is known that faces sliding on each other are subjected to severe wear in an early stage (hereinafter, referred to as “initial affinity”). The surface of a metal product treated by known shot blasting, due to the relatively high surface roughness thereof, requires an extended period of time for this initial affinity, and furthermore, causes a large change in size due to this initial affinity, decreasing the accuracy and efficiency.
In addition, although the structures in the surface layer of a metal product can also be miniaturized by the known shot ejection disclosed in '2433 (column [0017] of '2433), it has also been found that the known ejection is problematic in that the dimples whose shape are distorted due to wear, disappear over time because this microstructure at the surface layer is not satisfactorily uniform and is not robust in some cases, thus making it impossible to maintain the oil film.